This invention relates to a lens driving device and, in particular, to a lens driving device which is capable of moving a lens holder (a movable portion) for holding a lens assembly (a lens barrel) in the direction of an optical axis.
The portable compact camera is mounted in a camera-equipped cellular mobile phone. The portable compact camera is provided with an autofocus lens driving device. Previously, various autofocus lens driving devices have been proposed. As a driving source (a driving method) for use in such as a lens driving device, a VCM method using a voice coil motor (VCM) is known. The lens driving device of the VCM method comprises, as the driving source, a driving coil and a magnetic circuit which is composed of a yoke or yokes and permanent magnets.
By way of illustration, Japanese Unexamined Patent Application Publication No. 2009-86139 (JP-A-2009-86139) (which may also be called Patent Document 1) discloses a camera module in which a lens driving portion comprises a quadrilateral-shaped coil around an optical axis of a group of pick-up lenses, two flat-shaped magnets disposed to two opposed sides of the coil, and two yokes.
However, in the camera module disclosed in the Patent Document 1, there is a problem where it is impossible to raise efficiency of the magnetic circuit because the two yokes are disposed to the opposed sides of the coil.
On the other hand, Japanese Unexamined Patent Application Publication No. 2009-205042 (JP-A-2009-205042) (which may also be called Patent Document 2) discloses a lens driving device comprising, as a driving source, an octagonal driving coil, a yoke including an octagonal outer cylindrical portion, and a permanent magnet composed of eight rectangular permanent magnet pieces. In the lens driving device disclosed in Patent Document 2, inasmuch as the yoke includes the octagonal outer cylindrical portion, it is possible to suppress a reduction of efficiency of a magnetic circuit.
However, in the lens driving device disclosed in the Patent Document 2, there is a problem where a parts count is much and a mounting man-hour increases because the permanent magnet is composed of the eight rectangular permanent magnet pieces.
For this purpose, the present inventors (the present applicant) already created (made a suggestion) about a related lens driving device comprising a magnetic circuit including a permanent magnet composed of four rectangular (flat-shaped) permanent magnet pieces, in the manner which will later be described in conjunction with FIGS. 1 and 2.
In the related lens driving device, the magnetic circuit comprises a yoke including an outer hollow cylindrical portion having a rectangular hollow cylindrical shape.
In the related lens driving device, each flat-shaped permanent magnet piece is disposed with both ends thereof apart from opposed two sides of the outer hollow cylindrical portion of the yoke. In addition, the driving coil is apart from the flat-shaped permanent magnet piece in proximity to the both ends of the flat-shaped permanent magnet piece. As a result, there is a problem where a magnetic efficiency of the magnetic circuit reduces.
In addition, inasmuch as the related lens driving device uses, as the magnetic circuit, the permanent magnet composed of the four flat-shaped permanent magnet pieces, a component count increases. As a result, there are problems where it increases in costs and a mounting man-hour increases.